Activated Adsorption of Hydrogens on Aromatic—Alkali-Metal Charge-Transfer Complexes

Abstract

The ortho—parahydrogen-conversion reaction (p-H2⇌o-H2) and the hydrogen—deuterium exchange reaction (H2+D2⇌2 HD) were used to investigate the activated adsorption of hydrogen on aromatic—alkali-metal charge-transfer complexes. When deuterium gas was introduced onto these complexes, it was found that hydrogen exchange takes place between deuterium and the complexes (denoted as H2⇌D2). The kinetics of the parahydrogen conversion and of the H2—D2 exchange were studied as a function of the temperature and it was found that the rate of reactions was first order at a constant pressure. On the basis of the effect of the temperature on the rate constant, an apparent activation energy E for p-H2⇌o-H2 was 4.4 kcal/mole in the presence of phthalonitrile—Na (I), 3.7 kcal/mole with tetracyanopyrene—Cs (II), 10.0 kcal/mole with violanthrene B–Cs (III) and 0.6 kcal/mole with graphite—Cs (IV). Further, E for H2+D2⇌2 HD was 6.6 kcal/mole in the presence of (I) and ∼0.6 kcal/mole with (IV). These reaction suggests that the strong activated adsorption of hydrogen on the complexes takes place with the excess charge transferred from alkali metal to the organic molecule. Further, it was found that the hydrogen exchange reaction between gaseous hydrogen and phthalonitrile takes place reversibly.